A bicycle frame is a form of structural truss, the purposes of which are, first, to support the rider by transmitting his or her weight to the axles of the wheels and, second, to maintain the rear changer at a substantially fixed position relative to the chainrings for efficient transmission of the driving force through the chain.
The frame of a modern bicycle comprises a substantially triangular main section, termed the main triangle and composed of the seat tube, top tube, head tube, and down tube, and a "rear triangle" section composed of the seat tube, a pair of chainstays and a pair of seat stays. This frame geometry is highly efficient in supporting vertical loads but distinctly inefficient in supporting side loads. The vertical loads are transmitted primarily by tension and compression forces in the frame members while the side loads are transmitted by flexural and torsional forces. It is universally recognized that the inherent load-carrying characteristics of a bicycle frame are to a considerable extent somewhat the opposite of what is desirable. It would be preferable to have less stiffness in the vertical direction for improved comfort to the rider afforded by greater absorption in the frame of shock loads coming from the road and greater stiffness in the lateral direction for greater efficiency in the utilization of the energy of the rider and easier steering control and handling.
When a bicycle rider is pedaling even moderately hard, the frame rocks cyclically from side to side in correspondence with the shift of part of the rider's weight from one pedal to the other. This rocking motion creates side loads on the frame that bend the frame back and forth from side to side. With some bicycles the bending can be so great under very hard pedaling that the chain actually comes off the chainring due to loss of proper chain tracking. In any case, part of the rider's energy is transduced into frame deformation. Additional energy and attention are required for steering.
The rear changer and the chainrings are located about two inches off the center line of the frame. This results in torsional and lateral flexural loads in the rear triangle due to lateral deflection of the rear dropouts relative to the bottom bracket shell caused by the bending moment arm through which the chain forces act on the rear triangle. These loads are usually cyclical, and the dynamic effect of the changes during each crank cycle intensifies the loading, as compared to a static load.
The rear triangle is singularly important to the ability of the bicycle to carry as stably as possible a rear rack load. Bicycle tourists routinely transport many tens of pounds of gear on a rear rack. For this they need, first of all, a very stiff rack, lest this heavy "tail" start wagging the "dog" from time to time. Given a good rack, a heavy rack load introduces additional loads into the rear triangle that tend to produce side sway or "fish-tailing". If the rear triangle lacks lateral stiffness, another set of rider fatigue factors, those of greater steering effort and body control to offset the effects of a load that has, so to speak, a mind of its own come into play. In worst case situations, such as high speeds on downhill curves, fish-tailing is downright dangerous.
An object of the present invention is to stiffen the rear triangle in the lateral direction.
A further object of the invention is to maintain high torsional strength in the rear triangle.
It is also an object of the invention to minimize the weight of the rear triangle.